The separation of sub‐populations of T and B lymphocytes

Abstract Lymphocytes from mouse spleen and mouse thymus were separated on the basis of buoyant density in continuous gradients of albumin. Density fractions were examined for their content of subclasses of lymphocytes, determined by the presence of specific cell surface antigens. Density distribution profiles were determined for each lymphocyte subclass. The following findings emerged: 1 A minor subpopulation of thymus cells, characterized by a relatively high level of H‐2 antigen and a low level of Θ antigen on the cell surface, and a relative resistance to corticosteroids, was markedly enriched in the light density regions of the gradient. 2 The B cell and T cell populations of spleen, determined by a cytotoxic assay with anti‐Θ antiserum, showed substantial density overlap, although limited regions enriched for either B cells or T cells were obtained. 3 There were substantial differences between mouse strains in the density characteristics of the individual lymphocyte classes. 4 Each antigenic type of lymphocyte showed pronounced density heterogeneity, with discrete peaks in the distribution profiles. 5 The density distribution of T cells in the spleen and in the thymus were similar. 6 The data could be explained by two separate but similar differentiation pathways for the two antigenic types of T cells. One pathway (“High H‐2 ‐ Low Θ”) commences in the thymus, but maturation to a typical dense, small lymphocyte is usually completed after emmigration to the periphery. A second pathway (“Low H‐2 ‐ High Θ”) involves similar differentiation stages, but is restricted to the thymus.